A method generally employed in the manufacture of hydrogen fluoride involves heating a mixture of fluorspar and sulfuric acid in a rotation furnace. The crude hydrogen fluoride gases leaving the furnace are scrubbed to remove entrained solids, cooled and condensed to form an initial crude product. The initial crude product formed which comprises at least 95 percent by weight of HF contains various undesirable impurities and these are removed by fractional distillation to give technical or industrial grade anhydrous hydrogen fluoride which has a purity of 99.95% HF or better. The industrial grade anhydrous hydrogen fluoride thus obtained still contains large quantities of undesirable impurities such as arsenic. The amount of arsenic impurity which is present in industrial grade anhydrous hydrogen fluoride depends on the arsenic impurity in the fluorspar from which anhydrous hydrogen fluoride is produced. The industrial grade anhydrous hydrogen fluoride generally contains about 50-500 ppm of arsenic impurity. The presence of arsenic impurity in anhydrous hydrogen fluoride at these levels is highly undesirable for many applications. Anhydrous hydrogen fluoride is used in the refining and chemical manufacturing industries and arsenic impurities in anhydrous hydrogen fluoride can poison the catalyst and contaminate the manufactured product which adversely affects the product quality. In the electronics industry, aqueous solutions of hydrogen fluoride are used as cleaning agents and etchants in the manufacture of semiconductors, diodes and transistors. A high degree of purity and very low levels of arsenic in anhydrous hydrogen fluoride are required to prevent minute quantities of arsenic impurity from remaining on the surfaces of said electronic industry products after they have been cleaned or etched with hydrogen fluoride.
Several processes have been proposed to remove arsenic from industrial grade anhydrous hydrogen fluoride. Among these are processes which involve multiple fractional distillation in fluoroplastic equipment. However, such methods are suitable only for the preparation of small quantities of high purity anhydrous hydrogen fluoride and are not considered to be economical for producing commercial quantities.
U.S. Pat. No. 3,166,379 discloses a method whereby high purity aqueous hydrogen fluoride is produced by treatment with an oxidizing agent such as permanganate or chromate or with an oxidizing agent and a halogen, preferably iodine, to convert arsenic impurity to a non-volatile compound. This process is suitable for the purification of aqueous hydrofluoric acid but suffers from the drawback of contamination with manganese and chromium compounds when anhydrous fluoride is distilled.
A solution to the above problem is claimed in U.S. Pat. No. 3,689,370 which describes a process involving the addition of an inorganic ferrous salt to anhydrous hydrogen fluoride after the permanganate or chromate treatment to reduce excess oxidizing agent. However, this process also results in a high level of iron contamination in the final product.
The iron contamination problem is solved in U.S. Pat. No. 4,032,621 which describes a process involving the addition of a heavy metal free reagent such as hydrogen peroxide to anhydrous hydrogen fluoride after the permanganate or chromate treatment to reduce excess oxidizing agent. This process is very sensitive and may not be convenient for continuous operation.
In the systems of the above patents, arsenic is removed from aqueous hydrofluoric acid using aqueous permanganate solutions. However, because of the corrosion problem in aqueous systems, a system of that kind is not desirable. The process control of continuous operation in such systems would also present problems. The addition of water could be overcome by addition of solid potassium permanganate to anhydrous hydrogen fluoride for arsenic removal. But this process is associated with a potential explosion hazard problem during plant operation.
W. Roland et al., in East German Patent No. 62,309 (June 20, 1968), claim removal of arsenic from 80-90% aqueous HF with 30% H.sub.2 O.sub.2 at 40.degree.-60.degree. C. They also claim that arsenic removal from anhydrous hydrogen fluoride could be accomplished by continuously adding anhydrous hydrogen fluoride and hydrogen peroxide solution of suitable concentration to the reactor maintained at 80-90% HF and 40.degree. to 60.degree. C. The reactor contents are distilled and absorbed in water to produce purified aqueous solution of HF product. This process is limited to production of aqueous solution of HF product and is not suitable for manufacture of purified anhydrous hydrogen fluoride.
U.S. Pat. No. 4,083,941 claims removal of arsenic and sulfate from anhydrous hydrogen fluoride with the addition of 0.7% H.sub.2 O.sub.2 and 0.6% methanol at 0.degree.-75.degree. C. But this has the disadvantage of introducing a foreign material to the process.
U.S. Pat. No. 4,491,570 claims arsenic removal from anhydrous hydrogen fluoride by treating with hydrogen chloride or a fluoride salt or both and then separating purified anhydrous hydrogen fluoride by distillation from the non-volatile arsenic compounds. Again, this process has the potential of introducing a foreign material into purified anhydrous hydrogen fluoride.
U.S. Pat. No. 3,687,622 describes a high pressure distillation process at a pressure of 115 psia or above to produce low arsenic anhydrous hydrogen fluoride. The main drawback of this process is the very high pressure required for the distillation.
In all of these prior art processes for arsenic removal, there is danger of introducing a foreign material to purified anhydrous hydrogen fluoride. A need, therefore, exists for a simple and economical process for arsenic removal on a continuous basis which does not introduce any appreciable quantities of foreign material to purified anhydrous hydrogen fluoride and which is safe from explosion hazards.